The invention relates to a process for obtaining a polyhydroxyalkanoate, synthesized intracellularly by a microorganism, from the cell material, and polyhydroxyalkanoate flocs.
Polyhydroxyalkanoates, in particular homopolymers and copolymers of D-(-)-3-hydroxybutyric acid (poly-HB) are synthesized and accumulated intracellularly by many microorganisms as a storage substance for energy and carbon, and they represent polyesters which have thermoplastic properties and which are biodegradable. Poly-HB can be prepared in good yields with the aid of microorganisms, for example following the procedure described in U.S. Pat. No. 4,786,598 or U.S. Pat. No. 4,957,861. Copolyesters of poly-HB such as, for example, copolyesters which consist of 3-hydroxybutyric acid units and 3-hydroxyvaleric acid units or, alternatively, other acid units, can be prepared by way of fermentation, for example by one of the procedures described in EP-A-0 052 459, EP-A-0 204 442, EP-A0 288 908, EP-A-0 304 293 or EP-A-0 274 151. The polyhydroxyalkanoates formed are integrated into the cell material of the microorganism and must be separated from the cell material, which is relatively difficult. One possible way of separation is extraction with the aid of a solvent, but carrying out the processes described to date also presents difficulties.
For instance, to make the polyhydroxyalkanoate which is integrated in the cell material of the microorganism more accessible to the action of the solvent, it is necessary to provide a separate step for breaking up or solubilizing the cell material upstream of the actual extraction step. For example, U.S. Pat. No.3,044,942 describes pretreatment of the cells with acetone, U.S. Pat. No.3,275,610 describes mechanical pretreatment of the cells by pounding the cells or shaking them with hard objects, EP-A-0,015,123, EP-A-0,124,309 or EP-A-0,168,095 describe spray-drying or pre-drying the microorganism cells in another way, for example by removing the water by means of azeotropic distillation, and EP-A-0,015,669 describes pretreatment of the cells by osmotic shock, ultrasound or lysis of the cell wall. Moreover, the separation of the undissolved cell material which has been pretreated in this manner from the dissolved polyhydroxyalkanoate presents great problems since the undissolved cell material clogs the filters as a jellylike substance, and it is very difficult to remove all the organic phase from the cell material. This is why EP-A-0 046 017 describes flocculation of the cell material before extraction with the aid of a combined alkali, acid and heat treatment to provide better separability, while CA Vol.108, 1988, Ref.73835j proposes the use of a filtering aid such as perlite or diatomaceous earth for separating the cell material from the solvent.
Surprisingly, a simple process has now been found in which there is no need to break up the cells and in which the cell residues can be removed without any complicated procedures and without difficulties, the polyhydroxyalkanoate simultaneously being isolated in the form of flocs which are easy to manipulate and which are surprisingly well suited for further processing into objects.